Mechanical seals are broadly divided into an unbalanced type which is used in a region where a fluid pressure of a sealed fluid is relatively low, and a balanced type which is used in a region where the fluid pressure thereof is relatively high. The mechanical seal having a balance ratio of more than 1 is called the unbalanced type, and the mechanical seal having the balance ratio of equal to or less than 1 is called the balanced type. In a case where the fluid pressure acts from a rotating ring side to a stationary ring side, the balance ratio is a value obtained by S2÷S1, where S1 is a sliding area between the stationary ring and the rotating ring, S2 is a pressure receiving area where the stationary ring receives a pressure effect of the rotating ring due to the fluid pressure.
In a case where the mechanical seal is configured as the balanced type, conventionally, a configuration in which a step is provided on a rotating shaft has been adopted (see Patent Literatures 1 and 2). With reference to FIG. 3, a balanced type mechanical seal according to a conventional example will be described. FIG. 3 is a schematic cross-sectional view showing a mounted state of the balanced type mechanical seal according to the conventional example.
A mechanical seal 500 is used to seal an annular gap between a rotating shaft 600 and a housing 700. The mechanical seal 500 includes a stationary ring 510 provided on the housing 700 side and a rotating ring 520 which rotates together with the rotating shaft 600. The rotating ring 520 is pressed toward the stationary ring 510 side by a pressing member such as a spring which is not shown in the figure. Accordingly, a tip surface of an annular convex portion 511 provided on the stationary ring 510 is kept in contact with the rotating ring 520. Consequently, in a case where the rotating ring 520 is rotating together with the rotating shaft 600, the tip surface of the annular convex portion 511 and an end surface of the rotating ring 520 on an opposite side to the sealed fluid (F) side slide relative to each other.
In addition, the rotating shaft 600 is provided with a step. That is, the rotating shaft 600 has a small diameter portion 610 and a large diameter portion 620. The stationary ring 510 is disposed on an outer peripheral surface side of the small diameter portion 610. Note that an annular gap between an outer peripheral surface of the stationary ring 510 and an inner peripheral surface of the housing 700 is sealed by a seal ring 810. In addition, the rotating ring 520 is disposed in a state in which a gap between an outer peripheral surface of the large diameter portion 620 and an inner peripheral surface of the rotating ring 520 is sealed by a seal ring 820.
With the configuration described above, it becomes possible to set the balance ratio to be equal to or less than 1. The point that the balance ratio can be set to be equal to or less than 1 will be described more specifically. A sliding area S1 between the stationary ring 510 and the rotating ring 520 is a contacting area of the tip surface of the annular convex portion 511 and the end surface of the rotating ring 520 on the opposite side to the sealed fluid (F) side. That is, in the cross-sectional view shown in FIG. 3, the sliding area S1 is an area defined by the entire circumference of a distance A1 which is a distance from an inner peripheral surface to an outer peripheral surface of the annular convex portion 511. On the other hand, in the cross-sectional view shown in FIG. 3, the pressure receiving area S2 where the stationary ring 510 receives the pressure effect of the rotating ring 520 due to the fluid pressure is an area defined by the entire circumference of a distance A2 which is a distance from the outer peripheral surface of the large diameter portion 620 of the rotating shaft 600 to the outer peripheral surface of the annular convex portion 511. This is because the sealed fluid does not exist on a radially inner side of the outer peripheral surface of the large diameter portion 620, while on a radially outer side of the outer peripheral surface of the annular convex portion 511, the pressures act on the rotating ring 520 from either side in an axial direction; hence the fluid pressure is not applied to the stationary ring 510 via the rotating ring 520.
As described above, by providing the step on the rotating shaft 600 to increase the sliding area S1 (i.e., making A1≧A2 in FIG. 3), it becomes possible to satisfy S1 S2. Accordingly, it becomes possible to set the balance ratio (S2÷S1) to be equal to or less than 1.
However, in the conventional art, as described above, it is not possible to set the balance ratio to be equal to or less than 1 without providing the step on the rotating shaft. Accordingly, for example, in a case where a manufacturer of the mechanical seal is different from a manufacturer of the rotating shaft, a problem arises in that the manufacturer of the mechanical seal may not be able to control the balance ratio. Consequently, there is a need to set the balance ratio to be equal to or less than 1 by only the configuration of the mechanical seal.